Waste water may be treated by gasification, for example by the aeration or oxygenation of sewage or other waste water containing organic matter degradable by the action of oxygen thereon. A wide range of treatment methods and apparatus has been used and proposed. Oxygen does not dissolve easily or quickly in water and it is therefore in principle desirable to utilize fine bubble aerators wherein the bubbles are less than 2 mm and desirably less than 1 mm in diameter. Smaller bubbles have a larger specific surface area for oxygen transfer into the liquid, and also rise more slowly through the liquid to give a longer time for the oxygen to transfer before the bubble reaches the liquids surface. Treatment plants are known comprising a treatment vessel with aerator devices submerged in the waste water to produce the bubbles.
It has also been proposed to provide a treatment plant where the treatment vessel contains a bed of loose material. Aeration then causes a degree of fluidization of the bed and sustains the growth of a population of microorganisms on the material of the bed. In the presence of dissolved oxygen the microorganisms convert the organic matter in the waste water to carbon dioxide, water and to more bulky cellular materials and sludge thus alleviating the biological oxygen demand (BOD). Under appropriate operating conditions they will also convert ammonia to nitrate compounds. The surplus sludge thus formed can pass out with the effluent for eventual separation and recycling if desired.
Problems of fouling and clogging of the aerator devices and any pipe work can be acute where they are buried or caged beneath a bed of loose material. Regular closing and draining of the plant for cleaning and unblocking or replacement of the aerators is inefficient and expensive due to the need also to move aside or remove the filter bed material to gain access to the buried aerators.
Our Application no. PCT/GB94/02795 (now publication WO/95/17351) describes a method and apparatus for treatment of waste water wherein such problems are mitigated, and in particular discloses loose particulate material for use as a fluidizable bed in the waste water treatment, said material being characterized by particles of a substantially inert mineral adhered to, coated on or coated by plastics material to provide a habitat for microorganisms effective in waste water treatment.
That Application also discloses a waste water treatment method characterized by allowing waste water to enter a treatment vessel containing a bed of such loose particulate material, and gasifying the bed and the waste water by means of gas bubbles emitted from one or more gasifiers disposed within the bed and adapted for placement and removal from above the water level.
That Application also discloses apparatus for treatment of waste water characterized by a treatment vessel containing a bed of such loose particulate material, one or more gasifiers movably disposed within the bed, and means for supplying gas to the gasifiers for emission as gas bubbles to gasify the bed and the waste water, said gasifiers being adapted for placement and removal from above the water level.
That Application discloses such loose particulate material having a density in the range of from substantially 1.0 to substantially 1.3 g/cc, having a specific surface area in excess of approximately 600 m.sup.2 per cubic meter of the loose material, and having a particle size range of substantially 3 mm to substantially 10 mm in diameter. An example of the material is disclosed as particles of sand or gravel or other inert mineral particles at least partially adhered to, coated on or coated by plastics material, preferably a thermoplastics material such as polyethylene. It was disclosed that the material could be produced to a desired density for a particular application by changing the initial proportions of mineral and plastics. The particles may be formed by partially melting the polyethylene, for example in hot air, and allowing it to contact the sand or gravel. In use, the particles are quite loose and readily permit the gasifiers to be shaken down through the bed and placed where desired on the base of the vessel. No surrounding cage, grid or mesh was required.
Such loose particulate material has been found in practice to be effective for use as a fluidizable bed in waste water treatment by such methods and apparatus. Such materials have been found to provide a particularly suitable habitat for a high population density of microorganisms of the type effective in waste water treatment.